The basic element of a solid electrolytic capacitor is generally produced, as shown in FIG. 2, by forming a dielectric oxide film layer (2) on an anode substrate (1) comprising a metal foil subjected to an etching treatment to have a large specific surface area, forming a solid semiconductor layer (hereinafter referred to as a “solid electrolyte”) (4) as a counter electrode on the outer side of the dielectric oxide layer, and preferably further forming thereon an electrically conducting layer (5) such as electrically conducting paste. Using this element as-is alone or after stacking these elements, lead wires (6, 7) are connected thereto and the entirety is then completely encapsulated with sealing resin (8) such as epoxy resin and used as a part of a solid electrolytic capacitor (9) in electric products over a wide range.
With recent progress of digitization of electric devices or high-speed processing of personal computers, a compact and large-capacitance capacitor or a capacitor assured of low impedance in the high frequency region is being demanded. In order to meet this requirement, it has been recently proposed to use an electrically conducting polymer having electron conductivity as the solid electrolyte.
As for the technique of forming an electrically conducting polymer on a dielectric oxide film, an electrolytic oxidative polymerization method and a chemical oxidative polymerization method are generally known. In the chemical oxidative polymerization method, the reaction or the polymer film morphology is difficult to control, but the formation of solid electrolyte is easy and since this enables mass production in a short time, various methods have been proposed thereon. For example, a method of alternately repeating a step of dipping an anode substrate in a monomer-containing solution and a step of dipping the substrate in an oxidizing agent-containing solution, thereby forming a solid electrolyte having a layer structure, has been disclosed (see, Japanese Patent Publication No. 3,187,380; W099/28932). According to this method, a high-capacitance and low-impedance solid electrolytic capacitor with excellent heat resistance can be produced by forming a solid electrolyte layer having a layer structure with a film thickness of 0.01 to 5 μm. However, in this method it is necessary to form a thick solid electrolyte film to completely cover the inside of fine pores and outer surface of a capacitor element to exert such characteristics. In the light of use as an element for a multilayer capacitor fabricated by stacking a plurality of capacitor elements, it is demanded to further reduce the thickness of the solid electrolyte layer as a whole.
Various techniques have been disclosed for forming uniformly a solid electrolyte inside pores and on the outer surface of a capacitor element. For example, there is a disclosure of a method controlling the distribution of the attached amount of an oxidizing agent on the anode body during the process of forming an electrically conducting polymer to thereby control the thickness of an electrically conducting polymer (JP-A-2003-188052 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). It is also disclosed that forming a polymer layer on the circumference, especially on the corner part (JP-A-2001-143968).
Meanwhile, it has been disclosed that the short-circuit fraction defective can be reduced by removing burrs of an electrode foil (JP-A-1-251605 and JP-A-2004-296611). In JP-A-1-251605, a burr is referred to the part generated at the time of cutting on the outside of an electrode foil, and a method of shaping the cut edge of an electrode foil by pressing into the form of any combination of a wedge, convexity, almost-semiellipse, almost-R-shape and slope. Shaping by pressing leads to destroying the porous layer existing in the electrode foil, thereby degrading reliability. There is not any specific description with respect to the status of burrs after pressing, microstructure of the side surface of the electrode foil or the causes of generation of burrs on the electrode foil. Meanwhile, in JP-A-2004-296611, generation of burrs is prevented by chemically forming the valve-acting metal foil in which a cut has been made in advance and then physically cutting the foil. This method is effective in preventing the generation of burrs but a metal surface is newly generated on the cut end. Therefore, the valve-acting metal foil cannot be used as an electrode of a capacitor as it is, and requires to repeat cumbersome chemical formation treatment and is not a very economical method.